Olympic rings made up of 25,000 flowers are seen at Kew Gardens in London April 18, 2012 Photo: REUTERS

London entered the final straight in the run-up to the 2012 Olympics during a wet and windy spring day on Wednesday with celebrations across the world to mark the 100 days' countdown to the Games.

The day began at Kew Gardens where London organising committee chairman Sebastian Coe helped plant one of 40 oak trees which will be installed around the country to recognise Britain's role in the birth of the modern Olympic movement.

The trees were grown from acorns taken from an oak planted in 1890 by the founder of the Games, French aristocrat Baron Pierre de Coubertin, at the Much Wenlock Games in the county of Shropshire.

Twenty thousand flowers in the shape and colour of the five Olympic rings were revealed and a giant sandcastle, also featuring the rings, has been built at Weymouth and Portland, venue for the sailing events. More than 250 guardsmen formed the figure 100 at the Horse Guards Parade where the beach volleyball will be staged.

"These celebration are actually being marked around the globe today," Coe told a news conference. "From Times Square in New York through to Palestine, New Zealand and Socchi.

"If ever I needed reminding that we are delivering a Games for the whole world, it's actually been on my international engagement programme over the last few months.

"It's taken me effectively to five continents. I have rarely witnessed the level of excitement amongst national Olympic committee, amongst the media and certainly amongst the elite level competitors that are going to come and dignify our Games in 100 days' time.".

Unsurprisingly, the organising committee and government ministers remained resolutely upbeat about the third Games to be hosted by London following the 1908 and 1948 editions.

"SAFE AND SECURE GAMES"

Olympics Minister Hugh Robertson opted to field a question on security, the most burning issue after four suicide bombers killed 52 people on three packed underground trains and a bus the day after London was awarded the Games on July 6, 2005.

"Terrorism is a threat for any major western country," Robertson said. "It is now and it always has been.

"At the end of last year we rearranged the security budget precisely to meet that threat. There's now a much better mix of military, private security guards, volunteers backed by a very considerable military contingent who work very, very closely with the metropolitan police and the security services.

"I am absolutely as confident as I possibly can be 100 days out that we will deliver a safe and secure Games."

Culture, Olympics, Media and Sport Secretary of State Jeremy Hunt would not say whether any Syrian officials would be banned, although he said all European Union and United Kingdom travel bans remained in place.

"We are being very careful to make sure who we'll allow to come to these Games," Hunt said. "But in the end there will be people who come to these Games whose political views we disagree with," he said.

"We will be keeping a very, very close watch on who is coming in and will not allow people to come into the United Kingdom if we think there will be any kind of danger to national security."

Asked about the possibility of strike action affecting the Games, Hunt said the government was confident the vast majority of union members would want to make sure the Games were not interrupted.

"I don't think the unions will want to be on the wrong side of public opinion on that," he said. "So we are confident where there are industrial disputes to negotiate in good faith with the unions and we are confident in the end they will want to do the right thing."

Organising committee chief executive Chris Deighton said special measures had been put in place for the busiest day at London's main airport Heathrow.

Deighton said a special terminal would be in place to process departing athletes on August 13, the day after the closing ceremony.

The organising committee announced the aerobatic display team the Red Arrows would perform a flypast over Britain to mark the opening ceremony on July 27.

It also said 22 sites throughout Britain would show live sporting action on giant television screens from the Games to up to half a million a people a day.